This invention relates to a hot strip plant including a reversing roughing mill and a reversing finishing mill and a hot strip production method. More particularly, the present invention relates to a hot strip plant and a production method for producing efficiently a high quality sheet steel using a compact plant.
In a conventional compact hot strip plant including one reversing roughing mill and one reversing finishing mill equipped with a furnace coiler on its entrance and exit sides, the roughing mill and the finishing mill are disposed at a distance such that the material in the final pass of the roughing mill does not reach the finishing mill lest the roughing pass and the finishing pass are carried out simultaneously. Though this arrangement has such features that roughing and finishing can be carried out separately, it poses the following problems. First of all, the conveying distance of the material from the roughing mill to the finishing mill is long. Secondly, since many passes must be conducted in the finishing mill, a large heat loss cannot be avoided even when a furnace coiler is disposed on both entrance and exit sides of the finishing mill and particularly in the case of a thin material, a necessary final rolling temperature cannot be secured. Thirdly, descaling must be stopped in a pass where the thickness of the material is reduced below a predetermined thickness in order to avoid such a heat loss, which results in insufficient scale removal from the material and poor product quality.
To avoid these problems, U.S. Pat. No. 4,497,191 proposes a hot rolling mill of a web-like or sheet-like material. In accordance with this prior art technology, a 2-tandem reversible finishing mill which comprises two finishing mills each having coilers in front and at the back thereof is disposed in the proximity of a reversing roughing mill and selective tandem rolling by the roughing mill and two finishing mills and 2-tandem reversing rolling by two finishing mills are carried out in order to avoid the heat loss that would be caused by the reduction of the distance between the rolling mill and the finishing mills and by the 50% reduction of the number of take-up operations in the finishing mills and thus to obtain a high quality product.
To accomplish highly efficient production in a hot strip plant, it is very important how the system of the plant is interconnected as a whole. In conjuction with this point, a continuous casting plant and a hot rolling plant are connected to each other generally through a reheating furnace or a soaking pit furnace. Recently, direct rolling which rolls continuously a cast slab has been put into practical application and the has succeeded in saving energy. In any of these prior art technologies, however, a slab yard requiring a large space inside the continuous casting plant must be provided, and a rolling plant connected directly to a thin slab (about 50 mm thick) continuous casting machine has been proposed recently as a compact strip plant.
However, the plant disclosed in U.S. Pat. No. 4,497,191 described above involves the following problems. To begin with, since the roughing mill and the finishing mill are disposed adjacent to each other in this prior art technology, the material moves while passing through the roller gap of the finishing mill during a roughing pass and at this time, the surface of the material comes into contact with a sheet guide and rolls of the finishing roller. Therefore, the rolls are damaged due to the contact with the material, an oxide scale falls from the surface of the material and is deposited, this deposited scale is entrapped between the rolls at the time of finishing and causes damage to and surface deterioration of the rolls, and these defects are in turn transferred to the product and lower surface quality. To prevent these problems, the rolls must be ground very frequently and the rate of operation drops.
The fall of the scale in the finishing mill can be prevented by conducting descaling on the entrance side of the finishing mill. According to this method, however, only the portion of the material which enters the finishing mill is descaled and a localized temperature drop occurs at this portion. The temperature drop occurs also in the roughing pass because it is difficult to accomplish heat shield inside the finishing mill.
Furthermore, the finishing mill of this U.S. Pat. No. 4,497,191 employs the system for carrying out reversing rolling by disposing coilers in front and at the back of the two reversing rolling mills. Therefore, when finishing is carried out simultaneously by the two finishing mills, it is not possible to divide the finishing mill into a pre-stage portion and a post-stage portion, for example, so as to separate the material and to provide the material with rolling characteristics suitable for each pass that have been employed conventionally in a continuous hot strip tandem mill comprising six to seven rolling stands. In addition, quality of the product is deteriorated due to surface deterioration in view of the construction of the finishing mill. Furthermore, when reversing finishing is conducted by use of only one of the two finishing mills, productivity drops and at the same time, since the distance to one of the coilers is long, heat loss cannot be avoided.
Since the conventional compact strip plant is not equipped with means for accumulating slabs in addition to its difficulty in producing a thin slab, casting must be stopped when any trouble occurs in the rolling line, or unnecessary semifinished slabs are produced. In this sense, the system configuration is not rational. For these reason, a stable and highly efficient operation is difficult to perform.